Hipparchus, fl. 2d cent. b.c., Greek astronomer, b. Nicaea, Bithynia. He is the first systematic astronomer of whom there are records. He made his observations chiefly on the island of Rhodes. Ptolemy's geocentric theory of the universe was based largely on the conclusions of Hipparchus, a record of whose researches is preserved in the Almagest of Ptolemy. In it Hipparchus is credited with the discovery of the precession of the equinoxes, the eccentricity of the sun's apparent orbit, and certain inequalities of the motions of the moon. He determined the lengths of the seasons and accurately measured the year. He calculated the sizes of the sun and moon using eclipses. He also made the first known comprehensive chart of the heavens giving the positions of at least 850 stars, and he divided them into brightness classes, a system of magnitudes later expanded by Ptolemy. Hipparchus suggested a method of determining longitude by observing the parallax of the moon in eclipse. He is believed to have been the first to make systematic use of trigonometry, and he computed a table of chords roughly equivalent to trigonometrical sines. Only one of his works, a commentary on the work of Aratus and Eudoxus, survives.
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